The present invention relates to an apparatus for fabricating semiconductor devices, and a process for fabricating semiconductor devices. More specifically the present invention relates to a downflow-type plasma-treatment apparatus for use in the organic resist ashing and other treatments of processes for fabricating semiconductor devices.
Recently processes for fabricating semiconductor devices use plasma treatments using excited molecules, radicals, ions, etc. generated by molecular dissociation by plasma discharges. One of such plasma treatments is dry ashing for removing organic resists which are applied to wafers in lithography steps.
In this dry ashing, oxygen atom radicals, neutral particles dissociated from O.sub.2 (oxygen) plasma generated by using microwaves or radio frequencies (RF) are chemically reacted with an organic resist to remove the same. The ashing apparatuses for such dry ashing are divided in barrel-type RF plasma ashing apparatuses and downflow-type ashing apparatuses.
The conventional barrel-type RF plasma ashing apparatus will be explained with reference to schematic sectional view of FIG. 1.
A boat 42 is disposed in a vacuum chamber 40 of quartz through which microwaves are propagated. The vacuum chamber 40 has a gas feed port 44 for letting in O.sub.2 gas and a gas discharge port 46 for letting out the gas. On the outside of the vacuum chamber 40 there are disposed RF electrodes 48 for generating O.sub.2 plasmas in the vacuum chamber.
Si wafers with a resist applied to are loaded in the vacuum chamber 40 on the boat 42 to expose the wafers directly to O.sub.2 plasmas generated by the RF electrodes 48 for the ashing of the resists on the Si wafers 50 by ions, electrons and oxygen atom radicals.
But a problem with the barrel-type RF plasma ashing apparatus of FIG. 1 is that ions and electrons in the O.sub.2 plasmas have so high energies that they tend to cause damages by the radiation, contamination by Fe (iron), Na (sodium) to the devices formed on the Si wafers 50. To preclude such problem, the downflow-type ashing apparatus has been recently used.
Then, the conventional downflow-type ashing apparatus will be explained with reference to the schematic sectional view of FIG. 2.
This downflow-type ashing apparatus comprises a microwave guide for propagating microwaves 52, a plasma generating chamber 54 for generating O.sub.2 plasmas by the microwaves propagated by the microwave guide 52, and an ashing reaction chamber 56 for conducting ashing by oxygen atom radicals in the O.sub.2 plasmas. All the microwave guide 52, the plasma generating chamber 54 and the ashing reaction chamber 56 are formed of a metal, such as Al, or others so that the microwaves are blocked, and no contamination is caused.
The microwave guide 52 and the plasma generating chamber 54 are partitioned by a microwave transmitting window 58 of quartz for transmitting the microwaves. The plasma generating chamber 54 has a gas feed port 60 for letting in, e.g., O.sub.2 gas as an ashing gas.
The plasma generating chamber 54 and the ashing reaction chamber 56 are divided by an Al shower head 62 with a number of small holes opened in such a manner as to form a shower, so that the microwaves in the plasma generating chamber 54 are blocked, while only oxygen atom radicals, neutron particles in the O.sub.2 plasmas generated in the plasma generating chamber 54 are admitted into the ashing reaction chamber 56.
In the ashing reaction chamber 56, a stage 64 is disposed opposed to the shower head 62 at a certain distance from the shower head 62 for mounting Si wafers 66 to be treated.
Next, the ashing by the downflow-type ashing apparatus of FIG. 2 will be explained.
Si wafers 66 with a resist applied to the surfaces are mounted on the stage 54. Then O.sub.2 gas is fed into the plasma generating chamber 54 through the gas feed port 60 while microwaves are propagated into the microwave guide 52. The microwaves in the microwave guide 52 pass the microwave transmitting window 58 to glow O.sub.2 plasmas 68 in the plasma generating chamber 54.
Ions, electrons and oxygen atom radicals in the O.sub.2 plasmas 68 pass through the shower head 62 into the ashing reaction chamber 56. But because of a certain distance between the O.sub.2 plasmas 68 and the Si wafers 66 on the stage 64, substantially only oxygen radicals can arrive at the surfaces of the Si wafers 66. Then in the ashing reaction chamber 56 the oxygen atom radicals and the resist on the Si wafers 28 chemically react with each other, and ashing for removing the resist goes on.
In the conventional downflow-type ashing apparatus of FIG. 2, to block the transmission of the microwaves to the ashing reaction chamber 56 and prevent the contamination, the shower head 62 and the ashing reaction chamber 56 are used. The inside of the wall of the ashing reaction chamber 56 of Al is adversely heated up to 200.degree.-300.degree. C. under the influence of glowing of the O.sub.2 plasmas in the plasma generating chamber 54 in an ashing treatment. The shower head 62 of Al is more heated.
The oxygen atom radicals, the reaction seed of the ashing, are so unstable in their radical state that when they collide against the inside of the wall of the apparatus, the radicals extinguish with high probability and change into oxygen molecules in more stable state. The experiment conducted by the inventors using an ESR (electron spin resonance) apparatus showed that the extinction of the oxygen atom radicals, and temperatures of the inside of the wall of the apparatus on which the oxygen atom radicals collide have a relationship, and when the inside of the apparatus wall has a high temperature (about 200.degree.), the extinction of the oxygen atom radicals is accelerated, and the oxygen atom radicals do not easily extinguish when the inside of the apparatus wall has a low temperature (50.degree. C.).
Then ashing rates for temperature changes of the inside of the wall of the ashing reaction chamber 56 are measured as shown in the graph of FIG. 3. That is, there is a tendency that ashing rates decrease in proportion with rises of temperatures of the inside of the wall of the ashing reaction chamber 56. This tendency is found both in the case that O.sub.2 gas is used as the ashing gas and in the case a (O.sub.2 +H.sub.2 O) mixed gas of O.sub.2 gas and H.sub.2 O gas is used.
In other words, in the conventional downflow-type ashing apparatus, since the ashing is conducted by only oxygen atom radicals, an ashing rate varies depending on temperatures of the inside of the wall of the apparatus. When a temperature of the inside of the wall of the apparatus is low, a high ashing rate is obtained, and a low ashing rate is obtained when a temperature of the inside of the wall of the apparatus is high.